1. Field of the Invention
This invention relates to thrust reduction and flow control apparatus and methods in exhaust nozzle liner regions of gas turbine engines.
2. Description of the Prior Art
Since the 1950's, jet aircraft engine manufacturers have expended considerable effort toward developing variable cycle engines. One of the reasons for this effort and a key feature contributing to the high performance of a variable cycle turbojet engine, is its capability of maintaining inlet airflow as thrust is reduced. This feature leads to important performance advantages under less than maximum thrust conditions, such as during subsonic cruise. The effect of maintaining inlet airflow, as thrust is reduced, is to decrease such performance penalties as inlet spillage drag and afterbody closure drag, both of which have considerable effect.
Certain variable cycle engines, such as that described in U.S. Pat. No. 4,068,471, assigned to the same assignee as the present invention, achieve relatively constant airflow as thrust is varied by changing the amount of fan bypass flow with a valve system referred to as a variable area bypass injector (VABI). As engine thrust is decreased, this valve system (VABI) increases bypass flow to offset decreasing core engine flow resulting in a relatively constant total engine flow.
While the variable cycle engine attains performance increases by maintaining constant inlet flow while varying thrust, special considerations are encountered when an afterburner-type thrust augmentor is used. When thrust augmentation is employed on a conventional turbofan engine, greater thrust levels can be achieved by combining and mixing the bypass flow and core flow upstream of the augmentor burner. In the case of a variable cycle engine with variable bypass duct flow, a variable area injecting and mixing device (rear VABI) is used at the augmentor upstream of the engine exhaust nozzle. This rear VABI is required so that pressures and velocities are matched and orderly flow conditions established as the varying bypass flow is combined with the core flow for heating in the augmentor prior to being accelerated by the exhaust nozzle.
In the operation of a variable cycle engine with its bypass duct valving, conditions are encountered at the aft valve or rear VABI where a relatively large pressure differential exists across the valve. Since the best shield or liner used to contain the hot augmented gases is cooled by air from the bypass duct, the liner would be subjected to this high pressure differential. This is an unsatisfactory condition because high air pressure outside the liner would cause excessive stress. The liner would have to be structurally reinforced with an accompanying weight increase. In addition, the pressure differential would cause excessive cooling airflow through the liner resulting in a significant performance loss.
Another problem associated with engines employing thrust augmentation occurs during taxi operation of the aircraft when the engine is maintained at idle. Idle thrust of such engines is generally higher than desirable for easy aircraft handling during taxi conditions. In the variable cycle engine with a rear VABI, the injection of fan airflow through the engine exhaust nozzle is greatly diminished during taxi operations by closing the VABI valves which, to some extent, reduces engine thrust. However, much of the fan airflow is redirected through the heat shield or liner and eventually out the engine exhaust nozzle. Undesirable amounts of forward thrust are still produced by the variable cycle engine.